A known energy output device with multiple energy output sources including fuel cells is a power supply device having a secondary battery and fuel cells. In this power supply device, the control procedure stops power generation of the fuel cells and supplies electric power only from the secondary battery, when a load receiving a supply of electric power from the power supply device is in a preset low loading state. This control is based on the characteristic of the fuel cells that the total energy efficiency of the fuel cells is lowered in the low loading state. The control of stopping the operation of the fuel cells and activating only the secondary battery in the low loading state desirably enhances the overall efficiency of the power supply device.
At the time of a restart of the fuel cells in response to an increase of the load after the temporary stop of power generation of the fuel cells in the low loading state, however, there may be a disadvantage like a delayed response of power generation or a lowered voltage of a unit cell. Such disadvantage may be ascribed to, for example, condensation of water produced in the course of power generation in a gas flow path of the fuel cells or a partial decrease in moisture of electrolyte membranes in the case of polymer electrolyte fuel cells. The control of stopping power generation of the fuel cells based on the overall efficiency of the power supply device may thus not attain the theoretical high-efficient operation of the power supply device. The problem that a stop of power generation of the fuel cells may cause the above disadvantage at a restart of the fuel cells is commonly found in the control of stopping power generation of fuel cells under a preset condition in a general energy output device with multiple energy output sources including fuel cells.